1. Field of the Invention
The present invention relates to an ink jet recovery method which is carried out to recover the safe ink discharging condition of all ink jet recording head in the event of a discharging failure or which is conducted as a preventive measure for preventing occurrence of such a discharging failure. The invention also is concerned with an ink jet recording apparatus to which the recovery method is applied.
2. Description of Related Arts
In general, an ink jet recording apparatus is a device which records characters or patterns by means of tiny ink droplets discharged from an ink jet recording head. This type of recording device is superior in that it can output a fine and delicate image at a high speed. Known ink jet recording apparatus, however, suffer from a problem in that the ink discharging performance deteriorated to cause ink discharging failure due to various reasons, such as clogging of ink discharge openings in the recording head or introduction of air bubbles or foreign matter into ink paths between the ink discharging openings and a common ink chamber.
This problem is serious particularly in ink jet recording apparatus of a type in which ink droplets are discharged as a result of a change in the ink pressure attributable to a change in the state of the ink effected by generation of a bubble, which is formed as a result of film boiling of the ink caused by heat derived from an electro-thermal converting element (referred to as "discharge heater"). Namely, in this type of ink jet recording apparatus generation of bubbles tends to become unstable due to deposition of dyes of the ink on the discharge heater. Ink discharging failure is caused also by stagnation of bubbles which takes place in specific portions of the ink passages including the aforementioned ink paths leading to the discharging openings, particularly when the temperature of the whole recording device has been raised as a result of long continuous operation.
Japanese Patent Laid-Open (Kokai) No. 62-240558 discloses an ink jet recording apparatus having means serving as a fluid-diode. This fluid-diode means includes an ink heating device separate from the discharge energy generating means which generates the energy for discharging ink droplets. The ink heating means is energized in synchronization with the operation of the discharge energy generating means so as to form a bubble of ink vapor in the ink passage thereby preventing the ink from being displaced in the direction opposite to the openings.
Japanese Patent Laid-Open(Kokai) No. 62-238775 discloses an ink jet recording apparatus in which the pressure of the ink in the ink paths is controlled by vapor of the ink generated by a heating means provided in an ink chamber.
These ink jet recording apparatus suffer from a disadvantage in that the discharge of ink droplets tends to be impaired by stagnation of ink vapor bubbles in specific portions of the ink paths due to unstable generation of bubbles caused by deposition of ink dyes on the heating means.
In order to overcome these problems, it has been proposed to remove stagnat of bubbles or other foreign matter in the ink paths or to prevent such stagnation by sucking the ink from the discharging openings or by purging the bubbles together with the ink from the discharging openings by pressurizing the ink in the ink passage. An ink jet recording apparatus is also known from Japanese Patent Laid-Open(Kokai) No. 62-109648, in which air is once forced into the ink passage from the discharge side and then discharged. In an ink jet recording apparatus disclosed in Japanese Patent Laid-Open (Kokai) No. 62-109655, air is introduced into an intermediate portion of the ink passage through,for example, a three-way valve so as to force out bubbles together with the ink.
However, mere pressurizing or suction cannot satisfactorily remove bubbles stagnating in stepped portions or corners of the ink passage where the flow resistance is comparatively large. Repetition of the pressurizing or sucking operations for completely expelling bubbles results in a large ink consumption. On the other hand, introduction of air from the discharge side or to an intermediate portion of the ink passage requires a special mechanism for introducing the air in a controlled manner, with the result that the size of the ink discharge recovery system becomes impractically large.